ADSS cables with high-performance optical fiber

ABSTRACT

Disclosed is an improved optical fiber that employs a novel coating system. When combined with a bend-insensitive glass fiber, the novel coating system according to the present invention yields an optical fiber having exceptionally low losses. 
     The coating system features (i) a softer primary coating with excellent low-temperature characteristics to protect against microbending in any environment and in the toughest physical situations and, optionally, (ii) a colored secondary coating possessing enhanced color strength and vividness. 
     The improved coating system provides optical fibers that are useful in all-dielectric self-supporting (ADSS) cables.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/267,732 for a Microbend-Resistant Optical Fiber (filed Nov.10, 2008, and published Jul. 9, 2009, as U.S. Patent ApplicationPublication No. 2009/0175583 A1), which itself claims the benefit ofU.S. Provisional Application No. 60/986,737 for a Microbend-ResistantOptical Fiber (filed Nov. 9, 2007), U.S. Provisional Application No.61/041,484 for a Microbend-Resistant Optical Fiber (filed Apr. 1, 2008),and U.S. Provisional Application No. 61/112,595 for aMicrobend-Resistant Optical Fiber (filed Nov. 7, 2008).

This application further claims the benefit of U.S. ProvisionalApplication No. 61/112,926 for ADSS Cables With Bend-Insensitive Fiber(filed Nov. 10, 2008), U.S. Provisional Application No. 61/177,996 for aReduced-Diameter Optical Fiber (filed May 13, 2009), and U.S.Provisional Application No. 61/248,319 for a Reduced-Diameter OpticalFiber (filed Oct. 2, 2009).

Each of the foregoing commonly assigned patent applications and patentpublication is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention embraces optical fibers possessing an improvedcoating system that reduces stress-induced microbending. The presentinvention further embraces the deployment of such optical fibers inall-dielectric self-supporting (ADSS) cables.

BACKGROUND OF THE INVENTION

Fiber to the premises/business/home (i.e., FTTx) provides broadband datatransfer technology to the individual end-user. FTTx installations,which are being increasingly deployed throughout the world, are makinguse of innovative, reduced-cost system designs to promote the spread ofthe technology. For example, fiber may be delivered in the last link byway of a microcable. Air-blown fibers provide another efficient modelfor delivering the link to the end-use terminus. There continues to beindustry-wide focus on modes of deployment that overcome economicobstacles that impede fiber-based broadband solutions for datatransmission to businesses and residences.

Cost-effectiveness is important, of course, for achieving successfulFTTx systems. Reduced size for cables, drops, and structures for blowingare often critical, too. Installation of conduits suitable fortraditional cable designs is often prohibitive in existinginfrastructure. Thus, existing small ducts or tight pathways have to beused for new fiber installations. Low-cost and reduced-size requirementsare driving in a direction that reduces protection for the opticalfibers (i.e., away from conventionally robust, more bulky cabledesigns).

Glass designs are now available that offer reduced sensitivity to smallbending radius (i.e., decreased added attenuation due to the phenomenonknown as macrobending). These include trench-assisted core design orvoid-assisted fibers. Glass designs with lower mode field diameter areless sensitive to macrobending effects, but are not compatible with theG.652 SMF standard. Single-mode optical fibers that are compliant withthe ITU-T G.652.D requirements are commercially available, for instance,from Draka Comteq (Claremont, N.C.).

Microbending is another phenomenon that induces added loss in fibersignal strength. Microbending is induced when small stresses are appliedalong the length of an optical fiber, perturbing the optical paththrough microscopically small deflections in the core.

In this regard, U.S. Pat. No. 7,272,289 (Bickham et al.), which ishereby incorporated by reference in its entirety, proposes an opticalfiber having low macrobend and microbend losses. U.S. Pat. No. 7,272,289broadly discloses an optical fiber possessing (i) a primary coatinghaving a Young's modulus of less than 1.0 MPa and a glass transitiontemperature of less than −25° C. and (ii) a secondary coating having aYoung's modulus of greater than 1,200 MPa.

Nonetheless, better protection against microbending is still needed tohelp ensure successful deployment in more FTTx applications. To thisend, it is necessary to discover and implement new coating systems thatbetter address the demands FTTx installations place on fiber and cablestructures in a way that is commercially practical (i.e.,cost-effective).

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anoptical fiber having an improved coating system that provides improvedprotection against stress-induced microbending.

It is another object to provide an improved coating system that can bereadily mated with either single-mode optical fiber or multimode opticalfiber.

It is yet another object to provide an improved coating system that canbe readily mated with bend-insensitive optical fiber.

It is yet another object to provide an improved optical fiber coatingsystem including a primary coating that possesses a low modulus toprovide enhanced cushioning against lateral and axial stresses inducedby external forces.

It is yet another object to provide an improved optical fiber coatingsystem including a primary coating that possesses an exceptionally lowglass transition temperature (T_(g)) that reduces temperature-inducedstresses in unusually cold environments.

It is yet another object to provide an improved optical fiber coatingsystem including a primary coating that possesses an improved curingrate.

It is yet another object to provide an improved optical fiber coatingsystem including an ink-free secondary coating that has improvedbrightness and visibility.

It is yet another object to provide an improved optical fiber coatingsystem that can be applied at commercial processing speeds (e.g.,forming the primary coating at rates of at least about 20 meters persecond).

It is yet another object to provide an optical fiber possessing coatingsthat are readily stripped.

It is yet another object to provide an optical fiber having enhancedperformance characteristics for use in FTTx installations in whichconventional, robust cable designs are impractical.

It is yet another object to provide an optical fiber thatsynergistically combines a bend-insensitive glass fiber (e.g., DrakaComteq's single-mode glass fibers available under the trade nameBendBright^(XS)®) with the coating according to the present invention(e.g., Draka Comteq's ColorLock^(XS) brand coating system).

It is yet another object to provide an optical fiber that can beadvantageously deployed in buffer tubes and/or fiber optic cables.

It is yet another object to provide an optical fiber that requires lessexternal protection (e.g., enclosed within thinner buffer tubes and/orcable jacketing).

It is yet another object to provide a bend-insensitive optical fiberpossessing a reduced diameter (e.g., having thinner coating layersand/or a thinner component glass fiber).

It is yet another object to provide a reduced-diameter optical fiberthat requires less deployment space (e.g., within a buffer tube and/orfiber optic cable), thereby facilitating increased fiber count and/orreduced cable size.

It is yet another object to provide an optical fiber that can beinstalled in a way that employs small-radius bends.

It is yet another object to provide an optical fiber that facilitatesdirect installation onto buildings or other structures (e.g., stapled orotherwise secured to structural surfaces).

It is yet another object to provide a 200-micron single-mode opticalfiber that provides significantly better microbending performance thanthat of a standard single-mode optical fiber (SSMF) that employsconventional primary and secondary coatings (i.e., at an outer diameterof about 235-265 microns).

The foregoing, as well as other objectives and advantages of theinvention, and the manner in which the same are accomplished, arefurther specified within the following detailed description and itsaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts microbend testing results demonstrating thatexceptionally low microbending losses are achieved, in accordance withthe present invention, by pairing a bend-insensitive glass fiber with alow-modulus primary coating.

FIG. 2 schematically depicts the relationship between the in situmodulus of a primary coating and added loss for a multimode opticalfiber.

FIG. 3 depicts the dynamic mechanical properties of a typical commercialprimary coating (i.e., a conventional primary coating).

FIG. 4 depicts the dynamic mechanical properties of an exemplary primarycoating used in producing optical fibers according to the presentinvention.

FIG. 5 depicts microbend testing results for optical fibers that includea conventional primary coating and for optical fibers that include anexemplary primary coating according to the present invention.

FIG. 6 depicts microbend testing results (under rigoroustemperature-cycle testing conditions) for optical fibers that include aconventional primary coating and for optical fibers that include anexemplary primary coating according to the present invention.

FIG. 7 depicts microbend testing results (under modifiedtemperature-cycle testing conditions) for optical fibers that include aconventional primary coating and for optical fibers that include anexemplary primary coating according to the present invention.

FIG. 8 depicts microbend testing results demonstrating thatexceptionally low microbending losses are achieved, in accordance withthe present invention, by pairing a bend-insensitive glass fiber with alow-modulus primary coating.

FIG. 9 depicts microbend testing results (under rigoroustemperature-cycle testing conditions) for conventional optical fibersand for optical fibers that, in accordance with the present invention,combine a bend-insensitive glass fiber with a low-modulus primarycoating.

FIG. 10 depicts microbend testing results (under modifiedtemperature-cycle testing conditions) for conventional optical fibersand for optical fibers that, in accordance with the present invention,combine a bend-insensitive glass fiber with a low-modulus primarycoating.

FIG. 11 depicts attenuation (added loss) as a function of MAC number(i.e., mode field diameter divided by cutoff wavelength) for variousexemplary optical fibers.

FIG. 12 depicts, on a logarithmic scale, microbend sensitivity as afunction of MAC number (i.e., mode field diameter divided by cutoffwavelength) for various exemplary optical fibers.

FIG. 13 schematically depicts a cross-sectional view of an exemplaryADSS cable employing bend-insensitive optical fiber according to thepresent invention.

DETAILED DESCRIPTION

In one aspect, the present invention embraces optical fibers possessingan improved coating system that reduces stress-induced microbending,even in exceptionally cold environments required for FTTx deployments.The coating system according to the present invention includes a primarycoating that combines low in situ modulus (e.g., less than about 0.5 MPaas measured on the fiber) and low glass transition temperature (T_(g))(e.g., less than about −50° C.) to reduce stresses caused by externalforce and temperature. In addition, the coating system can be processedat high production speeds (e.g., 15-20 m/sec or more).

The present invention achieves a microbend-resistant optical fiber,particularly a single-mode optical fiber, by employing as its primarycoating a UV-curable, urethane acrylate composition. In this regard, theprimary coating includes between about 40 and 80 weight percent ofpolyether-urethane acrylate oligomer as well as photoinitiator, such asLUCIRIN® TPO, which is commercially available from BASF. In addition,the primary coating includes one or more oligomers and one or moremonomer diluents (e.g., isobornyl acrylate), which may be included, forinstance, to reduce viscosity and thereby promote processing. A suitablecomposition for the primary coating according to the present inventionis a UV-curable urethane acrylate product provided by DSM Desotech(Elgin, Illinois) under the trade name DeSolite® DP 1011.

In this regard, this application incorporates entirely by reference thefollowing commonly assigned patent application publications and patentapplications: U.S. Patent Application No. 60/986,737 for aMicrobend-Resistant Optical Fiber, filed Nov. 9, 2007, (Overton); U.S.Patent Application No. 61/041,484 for a Microbend-Resistant OpticalFiber, filed Apr. 1, 2008, (Overton); U.S. Patent Application No.61/112,595 for a Microbend-Resistant Optical Fiber, filed Nov. 7, 2008,(Overton); International Patent Application Publication No. WO2009/062131 A1 for a Microbend-Resistant Optical Fiber, (Overton); andU.S. Patent Application Publication No. US2009/0175583 A1 and itscounterpart U.S. patent application Ser. No. 12/267,732 for aMicrobend-Resistant Optical Fiber, (Overton).

This application further incorporates entirely by reference thefollowing commonly assigned patents, patent application publications,and patent applications: U.S. Pat. No. 4,838,643 for a Single Mode BendInsensitive Fiber for Use in Fiber Optic Guidance Applications (Hodgeset al.); U.S. Patent Application Publication No. US2007/0127878 A1 for aSingle Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No.7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S.Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (deMontmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic DispersionCompensating Fiber (de Montmorillon et al.); U.S. Pat. No. 7,555,186 foran Optical Fiber (Flammer et al.); U.S. patent application Ser. No.12/098,804 for a Transmission Optical Fiber Having Large Effective Area(Sillard et al.), filed Apr. 7, 2008; U.S. Patent ApplicationPublication No. US2009/0252469 A1 for a Dispersion-Shifted Optical Fiber(Sillard et al.); U.S. patent application Ser. No. 12/436,423 for aSingle-Mode Optical Fiber Having Reduced Bending Losses, filed May 6,2009, (de Montmorillon et al.); U.S. patent application Ser. No.12/436,484 for a Bend-Insensitive Single-Mode Optical Fiber, filed May6, 2009, (de Montmorillon et al.); U.S. patent application Ser. No.12/489,995 for a Wavelength Multiplexed Optical System with MultimodeOptical Fibers, filed Jun. 23, 2009, (Lumineau et al.); U.S. patentapplication Ser. No. 12/498,439 for a Multimode Optical Fibers, filedJul. 7, 2009, (Gholami et al.); U.S. patent application Ser. No.12/614,011 for a Reduced-Diameter Optical Fiber, filed Nov. 6, 2009,(Overton); and U.S. patent application Ser. No. 12/614,172 for aMultimode Optical System, filed Nov. 6, 2009, (Gholami et al.); U.S.Patent Application No. 61/101,337 for a Bend-Insensitive Optical Fiber,filed Sep. 30, 2008, (de Montmorillon et al.); U.S. Patent ApplicationNo. 61/112,006 for a Bend-Insensitive Single-Mode Optical Fiber, filedNov. 6, 2008, (de Montmorillon et al.); U.S. Patent Application No.61/112,374 for a Bend-Insensitive Single-Mode Optical Fiber, filed Nov.7, 2008, (de Montmorillon et al.).

One exemplary glass fiber, for instance, possesses a step-index corehaving a refractive index that is between about 0.003 and 0.006 higherthan the refractive index of its adjacent silica cladding.

Exemplary single-mode glass fibers for use in the present invention arecommercially available from Draka Comteq (Claremont, N.C.) under thetrade name BendBright®, which is compliant with the ITU-T G.652.Drequirements, and the trade name BendBright^(XS)®, which is compliantwith the ITU-T G.657.A/B and ITU-T G.652.D requirements.

In particular and as set forth herein, it has been unexpectedlydiscovered that the pairing of a bend-insensitive glass fiber (e.g.,Draka Comteq's single-mode glass fibers available under the trade nameBendBright^(XS)®) and a primary coating having very low modulus (e.g.,DSM Desotech's UV-curable urethane acrylate product provided under thetrade name DeSolite® DP 1011) achieves optical fibers havingexceptionally low losses (e.g., reductions in microbend sensitivity ofat least 10× (e.g., 40× to 100× or more) as compared with a single-modefiber employing a conventional coating system). Draka Comteq'sbend-resistant, single-mode glass fiber available under the trade nameBendBright^(XS)® employs a trench-assisted design that reducesmicrobending losses.

FIG. 1 depicts this outstanding result by comparing the aforementionedexemplary single-mode fiber according to the present invention withvarious single-mode fibers employing conventional coating systems. Inthis regard, FIG. 1 presents spectral attenuation data by measuringinitial spectral attenuation on the optical fiber on a shipping spool,thereby obtaining the peaks and valleys typical of the attenuationacross the full spectrum of wavelengths between the limits shown. Theoptical fiber is then wound onto a sandpaper-covered, fixed-diameterdrum (i.e., measurement spool) as described by the IEC fixed-diametersandpaper drum test (i.e., IEC TR62221, Method B), and another spectralattenuation curve is obtained.

The IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221, Method B)provides a microbending stress situation that affects single-mode fiberseven at room temperature. The sandpaper, of course, provides a roughsurface that subjects the optical fiber to thousands, if not millions,of stress points. With respect to the test data presented in FIG. 1, a300-mm diameter fiber spool was wrapped with adhesive-backed, 40-microngrade sandpaper (i.e., approximately equivalent to 300-grit sandpaper)to create a rough surface. Then, 400-meter fiber samples were wound atabout 2,940 mN (i.e., a tension of 300 gf on a 300-mm diametercylinder), and spectral attenuation was measured at 23° C.

The curves presented in FIG. 1 represent the difference between theinitial spectral curve and the curve when the fiber is on the sandpaperdrum, thereby providing the added loss due to microbending stresses.

* * *

Those having ordinary skill in the art will recognize cable designs arenow employing smaller diameter buffer tubes and less expensive materialsin an effort to reduce costs. Consequently, when deployed in such cabledesigns, single-mode optical fibers are less protected and thus moresusceptible to stress-induced microbending. As noted, the presentinvention provides an improved coating system that better protectsoptical fibers against stresses caused by external mechanicaldeformations and by temperature-induced, mechanical property changes tothe coatings.

As noted, conventional solutions for protecting optical fibers involvedusing large-diameter buffer tubes, buffer tubes made of high-modulusmaterials that resist deformation and stresses upon the fiber, andstronger, thicker cable jackets to resist deformations that might pinchor otherwise squeeze the optical fibers. These solutions, however, arenot only costly, but also fail to address the temperature-inducedstresses caused by changes to the protective coatings. In other words,conventional primary coatings possess high modulus at temperatures belowtheir respective glass transition temperatures.

As disclosed herein, the optical fiber according to the presentinvention includes a primary coating possessing lower modulus and lowerglass transition temperature than possessed by conventional single-modefiber primary coatings. Even so, the improved primary coatingformulation nonetheless facilitates commercial production of the presentoptical fiber at excellent processing speeds (e.g., 1,000 m/min ormore). In this regard, the primary coating employed in the opticalfibers of the present invention possesses fast curing rates—reaching 50percent of full cure at a UV dose of about 0.3 J/cm², 80 percent of fullcure at a UV dose of about 0.5 J/cm², and 90 percent of full cure at aUV dose of about 1.0 J/cm² as measured on a standard 75-micron film at20° C. and atmospheric pressure (i.e., 760 ton) (i.e., standardtemperature and pressure—STP).

* * *

FIG. 2 schematically depicts the observed relationship between the insitu modulus of a primary coating and the attenuation (added loss) ofthe optical fiber, here a 50-micron graded-index multimode fiber. Theprimary coating modulus is measured as cured on the glass fiber and theadded loss is measured using a fixed-diameter sandpaper drum procedurein accordance with the IEC TR62221 microbending-sensitivity technicalreport and standard test procedures (e.g., IEC TR62221, Method B, Ed.1), which are hereby incorporated by reference in their entirety.

As will be appreciated by those having ordinary skill in the art, prior,commercially available single-mode fibers typically include a Young'smodulus of 100-150 psi measured in situ (i.e., on the fiber). Theoptical fiber according to the present invention possesses a primarycoating having reduced modulus as compared with such commerciallyavailable primary coatings. Employing a lower modulus primary coatingprovides better cushioning around the glass fiber.

Although lower modulus of the in situ primary coating can be achieved byselectively undercuring, the present invention achieves in situ primarycoating having lower modulus even approaching full cure (i.e., near fullcure). In this regard, the modulus of the in situ primary coatingaccording to the present invention is less than about 0.65 MPa (e.g.,less than about 95 psi), typically less than about 0.5 MPa, and moretypically less than 0.4 MPa (e.g., between about 0.3 MPa and 0.4 MPa orbetween about 40 psi and 60 psi). It has been determined that an in situprimary coating having a modulus of less than about 0.5 MPasignificantly reduces bend sensitivity of the glass fiber. On the otherhand, the modulus of the in situ primary coating according to thepresent invention is typically greater than about 0.2 MPa (e.g., 0.25MPa or more).

To achieve its reduced modulus as compared with conventional opticalfiber coatings, the present primary coating possesses a lower crosslinkdensity, specifically a reduced concentration of the reactive acrylategroups. Those having ordinary skill in the art will appreciate thatacrylate groups crosslink via free radical polymerization duringphotoinitiation (e.g., UV-induced curing during drawing operations). Thereaction kinetics dictate reduced cure rates during processing. This iscommercially undesirable, of course, and so the present inventionimplements processing modifications to provide satisfactory cure ratefor the low-modulus primary coating.

There are at least two components of the curing process that retard therate of polymerization of the primary coating. First, the combination of(i) high curing temperatures induced by exposure to a high-intensity, UVenvironment and (ii) the exothermic polymerization reaction slows theobserved curing rate of the primary coating. Second, close proximity ofstacked UV lamps, in effect, creates rapidly superposed, repeatedphotoinitiation periods. The reaction rate of acrylate groups under thisconfiguration is likewise retarded—a somewhat counterintuitive result.With respect to the latter, disposing (i.e., positioning) UV lamps toincrease the period between consecutive UV exposures significantlyincreases the degree of coating cure as compared with other conventionalprocesses employing the same draw speed and UV dose. In this way, it ispossible to process the reduced-modulus, primary coating according tothe present invention in a way that achieves near-complete curing atfast fiber draw speeds, which are required for a commercially viableprocess. An exemplary method and apparatus for curing a coated fiber isdisclosed in commonly assigned U.S. Pat. No. 7,322,122, which is herebyincorporated by reference in its entirety.

The temperature dependence of the modulus is an important considerationto ensure that the primary coating provides enhanced microbendingprotection in FTTx applications. A primary coating having low modulusonly at room temperature would be inadequate because deployment in thefield will expose the optical fiber to microbend-inducing stresses atextreme environmental temperatures (e.g., −40° C. and below). Therefore,a suitable primary coating according to the present invention possessesan exceptionally low glass transition temperature so that the primarycoating remains soft and protective in extremely cold environmentalconditions.

* * * EXAMPLE 1 Comparison of Mechanical Properties

FIGS. 3 and 4, respectively, depict dynamic mechanical properties of atypical commercial primary coating (i.e., the conventional primarycoating) and an exemplary primary coating used in making the opticalfibers according to the present invention. The conventional primarycoating was a UV-curable urethane acrylate provided by DSM Desotech(Elgin, Ill.) under the trade name DeSolite® DP 1007. The exemplaryprimary coating according to the present invention (i.e., employed toform optical fibers of the present invention) was a UV-curable urethaneacrylate provided by DSM Desotech (Elgin, Ill.) under the trade nameDeSolite® DP 1011.

The data for the conventional primary coating were obtained on a DynamicMechanical Analyzer (DMA) at an oscillatory stress rate of 1 Hz. Indoing so, the strain was maintained within the linear region ofstress-strain behavior. The sample of conventional primary coating wascured on polyester to form a standard 75-micron film. A UV dose of 1J/cm² was applied using a mercury-halide bulb operating at a 300 W/inoutput. This UV exposure was sufficient to ensure that the coating wason the plateau of the dose-modulus curve.

Referring to FIG. 3, the data show the equilibrium modulus to beapproximately 1.5 MPa as measured on a 75-micron film. On a glass fiber(i.e., in situ), this conventional primary coating typically cures wellto a modulus of about 0.8 MPa, a level indicative of many single-modefiber primary coatings in the industry. Those having ordinary skill inthe art will appreciate that modulus measurements of softer primarycoatings tend to be lower on a glass fiber (i.e., in situ) as comparedwith on a 75-micron film.

The glass transition temperature of the conventional primary coating isestimated by the peak in tans to be approximately −30° C. Thus, theconventional primary coating (and similar formulations) will behave likea glassy polymer at extremely low temperatures (e.g., less than −40° C.,particularly less than −50° C.). (Although stress induced by strain istime dependent at low temperatures, estimated glass transitiontemperature is a useful comparative property.)

A sample of the exemplary primary coating according to the presentinvention was likewise cured on polyester to form a comparable 75-micronfilm. As before, a UV dose of 1 J/cm² was applied to the primary coatingusing a mercury-halide bulb operating at a 300 W/in output. As noted,FIG. 4 depicts dynamic mechanical properties of the exemplary primarycoating according to the present invention.

The exemplary primary coating according to the present inventionexhibited an equilibrium modulus at just under 1 MPa in the cured film.The in situ modulus (i.e., measured on the glass fiber), was betweenabout 0.3 MPa and 0.4 MPa. This is significantly lower than therespective modulus measurements for the conventional primary coating.

The glass transition temperature of the exemplary primary coatingaccording to the present invention is estimated by the peak in tans atless than about −50° C. (e.g., about −60° C.). This is at least about20° C. below the glass transition temperature of the comparative,conventional primary coating. Accordingly, primary coatings according tothe present invention provide much more rapid stress relaxation duringtemperature excursions.

* * *

As set forth in Examples 2 and 3 (below), two different methods wereused to evaluate the respective microbend sensitivities of glass fiberscoated with (i) a typical commercial primary coating (i.e., theconventional primary coating) and (ii) an exemplary primary coatingaccording to the present invention. As with Example 1 (above), theconventional primary coating was a UV-curable urethane acrylate providedby DSM Desotech (Elgin, Ill.) under the trade name DeSolite® DP 1007,and the exemplary primary coating according to the present invention(i.e., employed to form optical fibers of the present invention) was aUV-curable urethane acrylate provided by DSM Desotech (Elgin, Ill.)under the trade name DeSolite® DP 1011.

Each test method provided aggravated lateral stress conditions.Moreover, after measuring the effect on attenuation at room temperature,the test structures were temperature cycled to determine the additionalloss induced by such temperature excursions.

EXAMPLE 2 Comparison of Microbending Sensitivity

The first test method employed was a basket-weave, temperature cyclingprocedure known by those having ordinary skill in the art. According tothis test procedure, optical fiber was wound at about 490 mN (i.e., atension of 50 gf on a 300-mm diameter quartz cylinder with a 9-mm“lay”). Fifty layers were wound on the quartz drum to create numerousfiber-to-fiber crossovers. The testing procedure for Example 2 was anadaptation of IEC TR62221, Method D, which, as noted, is incorporated byreference in its entirety.

Those having ordinary skill in the art will appreciate that, at roomtemperature, such fiber crossovers can sometimes cause added loss (i.e.,if the optical fiber is very sensitive) but that typically little or noadded loss is observed. Consequently, the drum (with wound fiber) wastemperature cycled twice from about room temperature through (i) −40°C., (ii) −60° C., (iii) +70° C., and (iv) +23° C. (i.e., near roomtemperature) while making loss measurements at 1550 nanometers. In bothtemperature cycles, fiber attenuation was measured after one hour ateach test temperature.

FIG. 5 depicts exemplary results for single-mode glass fibers coatedwith, respectively, a conventional primary coating (i.e., DeSolite® DP1007) and an exemplary primary coating according to the presentinvention (i.e., DeSolite® DP 1011). The respective fiber specimens werechosen to match the coating geometry, mode field diameter, and cutoffwavelength. Accordingly, the respective optical fibers employeddifferent formulations of colored secondary coatings.

In summary, the conventional primary coating and the exemplary primarycoating according to the present invention each provided good protectionagainst microbending stresses at 23° C. Moreover, at −40° C., theoptical fiber having the conventional primary coating demonstrated onlya small added loss. (It would appear that at −40° C., the conventionalprimary coating provided adequate protection against microbending bystress relaxing in a reasonable timeframe, even though this was near itsglass transition temperature.) By way of comparison, the optical fiberaccording to the present invention demonstrated essentially no addedloss at −40° C. (i.e., better performance).

At −60° C., however, the optical fiber having the conventional primarycoating demonstrated significant added loss. (This temperature extremewas well below the glass transition temperature of the conventionalprimary coating.) By way of comparison, the optical fiber according tothe present invention demonstrated essentially no added loss at −60° C.,which is close to the glass transition temperature of this embodiment ofthe primary coating according to the present invention.

EXAMPLE 3 Comparison of Microbending Sensitivity

The second test method employed more aggressive environments (i.e.,conditions) in order to evaluate the respective microbend sensitivitiesof (i) an optical fiber possessing a typical commercial primary coating(i.e., the conventional primary coating) and (ii) an optical fiberpossessing an exemplary primary coating according to the presentinvention.

In particular, the second method modified the IEC fixed-diametersandpaper drum test (i.e., IEC TR62221, Method B), which, as noted, isincorporated by reference in its entirety, to provide a microbendingstress situation sufficiently harsh to affect single-mode fibers even atroom temperature (i.e., a rougher drum surface than that used to measurethe data depicted in FIG. 1). To do this, a 300-mm diameter quartz drumwas wrapped with adhesive-backed, 220-grit sandpaper (i.e.,approximately equivalent to 66-micron-grade sandpaper) to create a roughsurface.

In an initial test condition, each of the respective fiber samples waswound in a single layer at about 980 mN (i.e., a tension of 100 gf on a300-mm diameter quartz cylinder). In a modified test condition, three(3) each of the respective fiber samples was wound in a single layer atabout 1,470 mN (i.e., a tension of 150 gf on a 300-mm diameter quartzcylinder). Thus, as compared with the first test condition, the secondtest condition increased the winding tension by 50 percent.

Using matched fiber samples (as with the basket weave/temperaturecycling test of Example 2) fiber attenuation was measured after windingat room temperature (i.e., 23° C.) for each test condition. Then, thedrum (with 400 meters of wound fiber) was temperature cycled from aboutroom temperature through (i) −40° C., (ii) −60° C., and (iii) +23° C.(i.e., near room temperature) while making loss measurements at 1550nanometers using an optical time domain reflectometer (OTDR).

The several samples of each kind of optical fiber were initiallymeasured at 23° C. on the original spools (i.e., before winding on theroughened drum surface to establish baseline spectral attenuation) thenwere subjected to the foregoing rigorous testing conditions for one hourat each temperature. Fiber attenuation was measured after one hour (asin Example 2) at each test temperature.

FIG. 6, a line chart, and FIG. 7, a box plot, depict exemplary resultsunder these more rigorous testing conditions for single-mode opticalfibers that include a conventional primary coating (i.e., DeSolite® DP1007 UV-curable urethane acrylate) and for single-mode optical fibersthat include an exemplary primary coating according to the presentinvention (i.e., DeSolite® DP 1011 UV-curable urethane acrylate).

FIG. 6, for instance, shows that, as compared with conventional opticalfibers, exemplary optical fibers according to the present inventionpossess reduced microbend sensitivity (i.e., a reduction of about 40-60percent).

Likewise, FIG. 7 shows that, as compared with conventional opticalfibers, exemplary optical fibers according to the present inventionpossess substantially reduced microbend sensitivity at a higher windingtension (i.e., 150 gf on a 300-mm diameter quartz cylinder). FIG. 7 thusillustrates that the exemplary primary coating according to the presentinvention (i.e., DeSolite® DP 1011 UV-curable urethane acrylate)promotes both significantly reduced and significantly more uniformmicrobending performance.

* * *

In accordance with the foregoing, it has been found that, as comparedwith a conventional coating system, the present coating system providessignificant microbending improvement when used in combination with aconventional single-mode glass fiber.

It has been further found that pairing a bend-insensitive glass fiber(e.g., Draka Comteq's single-mode glass fibers available under the tradename BendBright^(XS)®) and a primary coating having very low modulus(e.g., DSM Desotech's UV-curable urethane acrylate product providedunder the trade name DeSolite® DP 1011) achieves optical fibers havingexceptionally low losses. Additional testing was performed, therefore,to demonstrate the dramatic and unexpected reductions in microbendsensitivity provided in accordance with the present invention.

EXAMPLE 4 Comparison of Microbending Sensitivity

The respective microbend sensitivities were measured for exemplaryoptical fibers, including (i) a conventional single-mode glass fiberwith a conventional commercial coating, (ii) a bend-insensitive glassfiber with a conventional commercial coating, and (iii) abend-insensitive glass fiber (e.g., Draka Comteq's single-mode glassfibers available under the trade name BendBright^(XS)®) with the coatingaccording to the present invention (e.g., Draka Comteq's ColorLock^(XS)brand coating system).

FIG. 8 demonstrates that the optical fiber according to the presentinvention, namely including a bend-insensitive glass fiber (e.g., DrakaComteq's single-mode glass fibers available under the trade nameBendBright^(XS)®) and a primary coating having very low modulus (e.g.,DSM Desotech's UV-curable urethane acrylate product provided under thetrade name DeSolite® DP 1011), provides exceptionally low attenuationlosses as compared with other optical fibers. Moreover, thisbend-resistant optical fiber exhibits small wavelength dependence withinthe transmission window between 1400 nanometers and 1700 nanometers, andis essentially unaffected by the microbend-inducing test conditionsacross the test spectrum.

FIG. 8 presents exemplary spectral attenuation data obtained adhering toIEC TR62221, Method B (fixed-diameter drum). In accordance with IECTR62221, Method B, initial spectral attenuation was measured on a440-meter sample of optical fiber wound on a shipping spool (i.e.,obtaining the peaks and valleys typical of the attenuation across thefull spectrum of wavelengths between the limits shown). The opticalfiber was then wound at about 3 N onto a 300-mm diameter measurementspool wrapped with adhesive-backed, 40-micron grade sandpaper (i.e.,approximately equivalent to 300-grit sandpaper), and another spectralattenuation curve was obtained.

Like the curves presented in FIG. 1, the curves depicted in FIG. 8represent, at 23° C., the difference between the initial spectral curveand the curve when the fiber is on the sandpaper drum of fixed diameter,thereby providing the added loss due to microbending stresses (i.e.,delta-attenuation across the spectral range).

EXAMPLE 5 Comparison of Microbending Sensitivity

The respective microbend sensitivities were measured under rigorous testconditions for exemplary optical fibers, including (i) a conventionalsingle-mode glass fiber with a conventional commercial coating and (ii)a bend-insensitive glass fiber (e.g., Draka Comteq's single-mode glassfibers available under the trade name BendBright^(XS)®) with the coatingaccording to the present invention (e.g., Draka Comteq's ColorLock^(XS)brand coating system).

FIG. 9 demonstrates that, even under extremely harsh conditions, theoptical fiber according to the present invention, namely including abend-insensitive glass fiber (e.g., Draka Comteq's single-mode glassfibers available under the trade name BendBright^(XS)®) and a primarycoating having very low modulus (e.g., DSM Desotech's UV-curableurethane acrylate product provided under the trade name DeSolite® DP1011), provides surprisingly low attenuation losses as compared withother optical fibers.

The testing procedure for Example 5 was an adaptation of IEC TR62221,Method B, which, as noted, is incorporated by reference in its entirety.For this modified IEC fixed-diameter sandpaper drum test, a 300-mmdiameter quartz drum was wrapped with adhesive-backed, 180-gritsandpaper (i.e., approximately equivalent to 78-micron-grade sandpaper)to create an even rougher surface than that described in Example 3(above). Then, 440-meter fiber samples were wound in a single layer atabout 1,470 mN (i.e., a controlled back tension of 150 gf on the 300-mmdiameter quartz cylinder using a Delachaux optical fiber windingapparatus), and spectral attenuation was measured.

FIG. 9 presents exemplary temperature-cycle data for three specimens ofstandard single-mode fiber (i.e., a conventional single-mode glass fiberwith a conventional commercial coating) and three specimens of opticalfiber according to the present invention (i.e., a bend-insensitive glassfiber with improved coating according to the present invention). Asnoted, 440 meters of optical fiber is wound onto the aforementionedsandpaper-covered, fixed-diameter drum. One hour after winding, fiberattenuation was measured at room temperature (i.e., 23° C.) using anoptical time domain reflectometer (OTDR). Then, the drum (with 440meters of wound fiber) was temperature cycled from about roomtemperature through (i) −40° C. and (ii) −60° C. in atemperature-controlled chamber. Fiber attenuation at 1550 nanometers wasmeasured by an OTDR after one hour of equilibration at both −40° C. and−60° C.

Microbending sensitivity (S_(m)) may be described as αR/T, wherein α isthe attenuation increase on the drum (dB/km), R is the radius of thefixed drum (mm), and T is the winding tension applied to the fiber (N).See e.g., IEC TR62221 Technical Report (Microbending Sensitivity). Inaddition to the parameters α, R, and T, however, themicrobending-sensitivity metric obtained from the fixed-diametersandpaper drum test is dependent on the coarseness of the sandpaperemployed on the measurement drum.

Table 1 (below) presents the microbending-sensitivity metric obtainedfrom the attenuation data (at a wavelength of 1550 nanometers) depictedin FIG. 9 (i.e., employing 180-grit sandpaper). Table 1 shows that, ascompared with a conventional standard single-mode fiber, the opticalfiber according to the present invention provides microbendingsensitivity that is about 2×-10× lower at 23° C. and about 2×-5× lowerat −40° C.:

TABLE 1 (Microbend Sensitivity) 23° C. −40° C. −60° C. Optical Fiber(dB/km)/ (dB/km)/ (dB/km)/ (Coating Color) (N/mm) (N/mm) (N/mm)Conventional SMF 139.9 220.6 331.8 (blue) Conventional SMF 261.0 329.7417.9 (red) Conventional SMF 104.3 161.9 228.0 (aqua) BendBright^(XS) ®w/ 35.8 76.5 163.4 ColorLock^(XS) (slate) BendBright^(XS) ® w/ 30.1 70.6144.2 ColorLock^(XS) (red) BendBright^(XS) ® w/ 42.7 84.7 166.4ColorLock^(XS) (aqua)

EXAMPLE 6 Comparison of Microbending Sensitivity

The respective microbend sensitivities were further measured forexemplary optical fibers, including (i) a conventional single-mode glassfiber with a conventional commercial coating and (ii) a bend-insensitiveglass fiber (e.g., Draka Comteq's single-mode glass fibers availableunder the trade name BendBright^(XS)®) with the coating according to thepresent invention (e.g., Draka Comteq's ColorLock^(XS) brand coatingsystem).

The testing procedure for Example 6 was an adaptation of IEC TR62221,Method B, which, as noted, is incorporated by reference in its entirety.For this modified IEC fixed-diameter sandpaper drum test, a 300-mmdiameter quartz drum was wrapped with adhesive-backed, 220-gritsandpaper (i.e., approximately equivalent to 66-micron-grade sandpaper)to create a rough surface like that described in Example 3. Each of thefiber samples was wound in a single layer at about 1,470 mN (i.e., atension of 150 gf on a 300-mm diameter quartz cylinder). As comparedwith the test conditions of Example 5, the test conditions of Example 6employed finer grade sandpaper (i.e., 220-grit rather than 180-grit).

As in Example 3, using matched fiber samples, fiber attenuation wasmeasured after winding at room temperature (i.e., 23° C.). Then, thedrum (with about 400 meters of wound fiber) was temperature cycled fromabout room temperature through (i) −40° C., (ii) −60° C., and (iii) +23°C. (i.e., near room temperature) while making loss measurements at 1550nanometers using an optical time domain reflectometer (OTDR).

Three (3) samples of each kind of optical fiber were initially measuredat 23° C. on the original spools (i.e., before winding on the rougheneddrum surface to establish baseline spectral attenuation) and then weresubjected to the foregoing rigorous testing conditions for one hour ateach temperature. Fiber attenuation was measured after one hour at eachtemperature.

FIG. 10 depicts exemplary results for single-mode optical fibers thatinclude a conventional primary coating (i.e., DeSolite® DP 1007UV-curable urethane acrylate) and for bend-insensitive glass fibers(e.g., Draka Comteq's single-mode glass fibers available under the tradename BendBright^(XS)®) that include a primary coating having very lowmodulus (i.e., DSM Desotech's UV-curable urethane acrylate productprovided under the trade name DeSolite® DP 1011).

FIG. 10 demonstrates that the optical fiber according to the presentinvention, namely Draka Comteq's single-mode glass fibers availableunder the trade name BendBright^(XS)® with a primary coating having verylow modulus (e.g., DSM Desotech's UV-curable urethane acrylate productprovided under the trade name DeSolite® DP 1011), provides exceptionallylow attenuation losses as compared with standard single-mode opticalfibers (SSMF).

In addition, FIGS. 11 and 12 depict attenuation and microbendsensitivity, respectively, at a wavelength of 1550 nanometers as afunction of MAC number (i.e., mode field diameter divided by cutoffwavelength) for various exemplary optical fibers in accordance with thestandard IEC fixed-diameter sandpaper drum test (i.e., IEC TR62221,Method B). The respective attenuation data depicted in FIG. 11 (addedloss) and FIG. 12 (microbend sensitivity) were obtained at 23° C. underthe test conditions previously described with respect to FIG. 1 (i.e.,400-meter fiber samples were wound at about 2,940 mN (i.e., a tension of300 gf) on a 300-mm diameter fiber spool wrapped with adhesive-backed,40-micron grade sandpaper).

FIG. 11 shows that Draka Comteq's bend-resistant, single-mode glassfiber available under the trade name BendBright^(XS)® in combinationwith Draka Comteq's ColorLock^(XS) brand coating system providesoutstanding performance with respect to added loss.

FIG. 12 shows that Draka Comteq's bend-resistant, single-mode glassfiber available under the trade name BendBright^(XS)® in combinationwith Draka Comteq's ColorLock^(XS) brand coating system providessuperior microbend sensitivity (i.e., microbend sensitivity of 0.01 to0.03 (dB/km)/(gf/mm)).

* * *

The optical fibers according to the present invention typically furtherinclude a tough secondary coating to protect the primary coating andglass fiber from damage during handling and installation. For example,the secondary coating might have a modulus of between about 800 MPa and1,000 MPa (e.g., about 900 MPa) as measured on a standard 75-micronfilm. As disclosed herein, this secondary coating may be inked as acolor code or, preferably, may be color-inclusive to provideidentification without the need for a separate inking process.

In one embodiment according to the present invention, the secondarycoating, which surrounds the primary coating to thereby protect thefiber structure, features an inclusive coloring system (i.e., notrequiring an extra layer of ink to be added for color coding). Thecolors, which conform to Munsell standards for optical fibercolor-coding, are enhanced for brightness and visibility under dimlighting (e.g., in deep shade or in confined spaces, such as manholes)and are easily distinguished against both light and dark backgrounds.

Furthermore, the secondary coating features a surface that provides anexcellent interface with ribbon matrix material so that the matrixseparates easily from the colored fiber in a way that does not sacrificerobustness. The mechanical properties of the colored secondary coatingare balanced with those of the primary coating so that, in heatstripping, the coating/matrix composite separates cleanly from the glassfibers.

* * *

Employing Draka Comteq's bend-resistant, single-mode glass fiberavailable under the trade name BendBright^(XS)® (or the trade nameBendBright-Elite™) with the present dual-coating system, which includesa low-modulus primary coating, has been found to reduce microbendingsensitivity by between about one to two orders of magnitude relative tostandard single-mode fiber (SSMF) at the key transmission frequencies of1550 nanometers and 1625 nanometers. As noted, such optical fiber notonly provides outstanding resistance to microbending and macrobending,but also complies with the ITU-T G.657.A/B and ITU-T G.652.Drequirements.

In particular, Draka Comteq's bend-resistant, single-mode glass fiberavailable under the trade name BendBright^(XS)® (e.g., enhanced withDraka Comteq's ColorLock^(XS) brand coating system) provides resistanceto macrobending required for sustained bends having a radius as low asfive (5) millimeters with an estimated failure probability of less thantwo (2) breaks per million full-circle bends (i.e., 360°) over 30 yearsin a properly protected environment. These bend-resistant optical fibersfacilitate the rapid deployment of small, flexible cables for thedelivery of fiber to the premises/business/home (i.e., FTTx) by virtueof the optical fiber's ability to sustain a loss-free transmissionthrough small-radius bends. Cables employing such bend-resistant opticalfibers may be routed around sharp bends, stapled to building frame,coiled, and otherwise employed in demanding environments while retainingclear and strong signal transmission.

* * *

In another aspect, the bend-insensitive optical fibers according to thepresent invention facilitate the reduction in overall optical-fiberdiameter. As will be appreciated by those having ordinary skill in theart, a reduced-diameter optical fiber is cost-effective, requiring lessraw material. Moreover, a reduced-diameter optical fiber requires lessdeployment space (e.g., within a buffer tube and/or fiber optic cable),thereby facilitating increased fiber count and/or reduced cable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the optical fiber according to the present invention,the component glass fiber typically has an outer diameter of about 125microns. With respect to the optical fiber's surrounding coating layers,the primary coating typically has an outer diameter of between about 175microns and about 195 microns (i.e., a primary coating thickness ofbetween about 25 microns and 35 microns) and the secondary coatingtypically has an outer diameter of between about 235 microns and about265 microns (i.e., a secondary coating thickness of between about 20microns and 45 microns). Optionally, the optical fiber according to thepresent invention may include an outermost ink layer, which is typicallybetween two and ten microns in thickness.

In one alternative embodiment, an optical fiber according to the presentinvention may possess a reduced diameter (e.g., an outermost diameterbetween about 150 microns and 230 microns). In this alternative opticalfiber configuration, the thickness of the primary coating and/orsecondary coating is reduced, while the diameter of the component glassfiber is maintained at about 125 microns. (Those having ordinary skillin the art will appreciate that, unless otherwise specified, diametermeasurements refer to outer diameters.)

In such exemplary embodiments, the primary coating layer may have anouter diameter of between about 135 microns and about 175 microns (e.g.,about 160 microns), typically less than 165 microns (e.g., between about135 microns and 150 microns), and usually more than 140 microns (e.g.,between about 145 microns and 155 microns, such as about 150 microns).Moreover, in such exemplary embodiments, the secondary coating layer mayhave an outer diameter of between about 150 microns and about 230microns (e.g., more than about 165 microns, such as 190-210 microns orso), typically between about 180 microns and 200 microns. In otherwords, the total diameter of the optical fiber is reduced to less thanabout 230 microns (e.g., between about 195 microns and 205 microns, andespecially about 200 microns).

One exemplary optical-fiber embodiment employs a secondary coating ofabout 197 microns at a tolerance of +/−5 microns (i.e., asecondary-coating outer diameter of between 192 microns to 202 microns).Typically, the secondary coating will retain a thickness of at leastabout 10 microns (e.g., an optical fiber having a reduced-thicknesssecondary coating of between 15 microns and 25 microns).

In accordance with the foregoing, a particular reduced-diameter,optical-fiber embodiment having exceptionally low losses employs DrakaComteq's 125-micron single-mode glass fiber available under the tradename BendBright^(XS)® with a 155-micron-diameter, low-modulus primarycoating layer (e.g., Draka Comteq's ColorLock^(XS) brand coating system)and a secondary coating (e.g., a nominal 200-micron-diameter secondarycoating). As noted, BendBright^(XS)® bend-insensitive optical fibercomplies with the ITU-T G.657.A/B and ITU-T G.652.D requirements. Inthis optical-fiber embodiment, the maximum tolerance with respect to theprimary-coating thickness is +/−5 microns (i.e., a primary-coating outerdiameter of between 150 microns and 160 microns), more typically about+/−2.5 microns (i.e., a primary-coating outer diameter of between about152.5 microns and 157.5 microns).

Another particular reduced-diameter, optical-fiber embodiment havingexceptionally low losses employs Draka Comteq's 125-micron single-modeglass fiber available under the trade name BendBright-Elite™ with a155-micron-diameter, low-modulus primary coating layer (e.g., DrakaComteq's ColorLock^(XS) brand coating system) and a secondary coating(e.g., a nominal 200-micron-diameter secondary coating). LikeBendBright^(XS)® bend-insensitive optical fiber, BendBright-Elite™bend-insensitive optical fiber complies with the ITU-T G.657.A/B andITU-T G.652.D requirements. In this optical-fiber embodiment, themaximum tolerance with respect to the primary-coating thickness is +/−5microns (i.e., a primary-coating outer diameter of between 150 micronsand 160 microns), more typically about +/−2.5 microns (i.e., aprimary-coating outer diameter of between about 152.5 microns and 157.5microns).

The synergistic combination of (i) Draka Comteq's BendBright^(XS)®bend-insensitive single-mode glass fiber (or Draka Comteq'sBendBright-Elite™ bend-insensitive glass fiber) and (ii) Draka Comteq'sColorLock^(XS) brand coating system promotes significant reductions inoptical-fiber diameter.

By way of example, Draka Comteq's 125-micron BendBright^(XS)®bend-insensitive single-mode glass fiber in combination with a155-micron-diameter, low-modulus primary coating layer (e.g., DrakaComteq's ColorLock^(XS) brand coating system) and a 200-micron-diametersecondary coating layer provides (i) comparable microbending performanceto that of a 125-micron, standard single-mode glass fiber coated with a185-micron-diameter, low-modulus primary coating layer (e.g., DrakaComteq's ColorLock^(XS) brand coating system) and a 242-micron-diametersecondary coating layer and (ii) significantly better microbendingperformance than that of a standard single-mode optical fiber (SSMF)that employs conventional primary and secondary coatings (i.e., at anouter diameter of about 235-265 microns).

As noted previously, one suitable composition for the primary coating isa UV-curable urethane acrylate product provided by DSM Desotech (Elgin,Ill.) under the trade name DeSolite® DP 1011. It is believed that thisUV-curable urethane acrylate product includes about 1.0 percent ofadhesion promoter. Other suitable compositions for the primary coatinginclude alternative UV-curable urethane acrylate products provided byDSM Desotech under various trade names, including DeSolite® DP 1014,DeSolite® DP 1014XS, and DeSolite® DP 1016. It is believed that thesealternative compositions possess essentially the same low-modulus andglass-transition properties as those possessed by the aforementionedDeSolite® DP 1011 UV-curable urethane acrylate product, albeit with somecompositional variation (e.g., adhesion promoter concentration increasedto 1.25 percent). As will be appreciated by those having ordinary skillin the art, compositional variations may provide particularprimary-coating properties that are desirable for particularapplications. It appears that the DeSolite® DP 1014XS UV-curableurethane acrylate product, for instance, exhibits favorable processingcharacteristics and provides improved delamination resistance.

Those having ordinary skill in the art will appreciate that each ofthese exemplary UV-curable urethane acrylate products (i.e., DeSolite®DP 1011, DeSolite® DP 1014, DeSolite® DP 1014XS, and DeSolite® DP 1016)provides better microbending performance than do conventional primarycoatings, such as other UV-curable urethane acrylate products providedby DSM Desotech under the respective trade names DeSolite® DP 1004 andDeSolite® DP 1007.

EXAMPLE 7 Comparison of Microbending Sensitivity

The respective microbend sensitivities were further measured forexemplary optical fibers, including (i) an enhanced single-mode glassfiber (ESMF) with a low-modulus coating, (ii) various bend-insensitiveglass fibers (e.g., Draka Comteq's single-mode glass fibers availableunder the trade names BendBright^(XS)® with conventional primarycoatings, and (iii) various bend-insensitive glass fibers andmacrobend-resistant glass fibers (e.g., Draka Comteq's single-mode glassfibers available under the trade names BendBright^(XS)® and BendBright®)with low-modulus primary coatings.

The testing procedure for Example 7 was an adaptation of IEC TR62221,Method B, which, as noted, is incorporated by reference in its entirety.For this modified IEC fixed-diameter sandpaper drum test, a300-millimeter diameter quartz cylinder was wrapped withadhesive-backed, 320-grit sandpaper (i.e., approximately equivalent to36-micron-grade sandpaper) to create a rough surface—albeit a finersurface than the surfaces employed in Examples 3-6. Then, each 440-meterfiber sample was wound in a single layer at about 1,470 mN (i.e., acontrolled tension of 150 gf on the 300-millimeter diameter quartz drumusing a Delachaux optical fiber winding apparatus). For the sake ofconvenience, this particular modification of the IEC TR62221, Method B,is herein referred to as the “Reduced-Diameter Optical-Fiber MicrobendSensitivity Test.”

Two hours after winding, fiber attenuation was measured at roomtemperature (i.e., 23° C.) using an optical time domain reflectometer(OTDR). Then, the drum (with 440 meters of wound fiber) was temperaturecycled in a temperature-controlled chamber from about room temperaturethrough (i) −40° C. and (ii) −60° C. Fiber attenuation was measured byan optical time domain reflectometer (OTDR) after two hours ofequilibration at both −40° C. and −60° C.

Absolute fiber attenuation measured at a wavelength of 1550 nanometersis provided (below) in Table 2.

TABLE 2 (Microbend Sensitivity - 1550 nm) Optical Fiber glass fiberw/primary coating 23° C. −40° C. −60° C. Ex. (glass fiber and coatingdiameters) (dB/km) (dB/km) (dB/km) 200-micron bend-insensitive SMFs withlow-modulus primary coatings A BendBright^(XS) ® w/DP1014XS 1.114 1.0191.002 (125μ/155μ/199μ) B BendBright^(XS) ® w/DP1014XS 1.786 1.612 1.542(125μ/150μ/199μ) C BendBright^(XS) ® w/DP1016 1.488 1.367 1.536(125μ/150μ/199μ) 200-micron bend-insensitive SMFs with conventionalprimary coatings D BendBright^(XS) ® w/DSM 950-076 2.726 3.215 3.595(125μ/160μ/199μ) E BendBright^(XS) ® w/DSM 950-076 4.288 4.766 5.150(125μ/150μ/199μ) 200-micron macrobend-resistant SMFs with low-modulusprimary coatings F BendBright ® w/DP1014XS 4.683 4.348 4.878(125μ/150μ/199μ) G BendBright ® w/DP1016 5.985 5.800 6.399(125μ/150μ/199μ) 242-micron enhanced SMF with low-modulus primarycoatings H ESMF w/DP1014 0.705 0.663 0.648 (125μ/190μ/242μ)

Table 2 (above) shows that Draka Comteq's 125-micron BendBright^(XS)®bend-insensitive single-mode glass fiber facilitates a reduction intotal optical-fiber diameter by permitting use of thinner primary and/orsecondary coatings. In this regard, a 200-micron optical fiber usingDraka Comteq's BendBright^(XS)® bend-insensitive single-mode glass fiberand relatively thin primary and secondary coatings provides microbendingperformance that approaches that of a 242-micron optical fiber having anenhanced standard single-mode fiber (ESMF) and thicker layers ofcomparable low-modulus primary and secondary coatings.

Absolute fiber attenuation measured at a wavelength of 1310 nanometersis provided (below) in Table 3:

TABLE 3 (Microbend Sensitivity - 1310 nm) Optical Fiber glass fiberw/primary coating 23° C. −40° C. −60° C. Ex. (glass fiber and coatingdiameters) (dB/km) (dB/km) (dB/km) 200-micron bend-insensitive SMFs withlow-modulus primary coatings A BendBright^(XS) ® w/DP1014XS 0.954 0.8690.758 (125μ/155μ/199μ) B BendBright^(XS) ® w/DP1014XS 1.574 1.426 1.478(125μ/150μ/199μ) C BendBright^(XS) ® w/DP1016 1.496 1.381 1.509(125μ/150μ/199μ) 200-micron bend-insensitive SMFs with conventionalprimary coatings D BendBright^(XS) ® w/DSM 950-076 2.238 2.683 3.015(125μ/160μ/199μ) E BendBright^(XS) ® w/DSM 950-076 4.020 4.363 4.671(125μ/150μ/199μ) 200-micron macrobend-resistant SMFs with low-modulusprimary coatings F BendBright ® w/DP1014XS 2.670 2.447 2.761(125μ/150μ/199μ) G BendBright ® w/DP1016 3.725 3.550 3.927(125μ/150μ/199μ)

The comparative 200-micron optical fiber designated Example D in Tables2 and 3 (above) employed the secondary coating used in Draka Comteq'sColorLock^(XS) brand coating system, albeit with a conventional primarycoating. The comparative 200-micron optical fiber designated Example Ein Tables 2 and 3 (above) employed both a conventional primary coating(i.e., DSM 950-076) and a conventional secondary coating (i.e., DSM950-044).

Tables 2 and 3 (above) indicate that, all things being equal, thelow-modulus primary coatings according to the present invention (e.g.,Draka Comteq's ColorLock^(XS) brand coating system) provide bettermicrobending performance than do conventional coating systems. Thissuperior microbending performance is especially important when employinga primary-coating layer at a significantly reduced thickness on a125-micron glass fiber in order to achieve a nominal 200-micron opticalfiber.

Moreover, Tables 2 and 3 (above) indicate that, all things being equal,Draka Comteq's single-mode glass fibers available under the trade nameBendBright^(XS)®, which employ a trench-assisted design, provide bettermicrobending performance than do single-mode fibers that do not employtrench-assisted and/or void-assisted design (e.g., Draka Comteq'ssingle-mode glass fibers available under the trade name BendBright®).This is somewhat unexpected trench-assisted and other bend-insensitiveglass designs are generally understood to have more pronounced effectsupon macrobending rather than microbending.

EXAMPLE 8 Comparison of Microbend Sensitivity

The respective microbend sensitivities were further measured inaccordance with the IEC fixed-diameter sandpaper drum test (i.e., IECTR62221, Method B) for exemplary optical fibers, including (i) enhancedsingle-mode glass fibers (ESMF) with Draka Comteq's ColorLock brandcoating system and (ii) Draka Comteq's single-mode glass fibersavailable under the trade name BendBright^(XS)® with Draka Comteq'simproved ColorLock^(XS) brand coating system.

As with Example 7 (above), the testing procedure for Example 8 waslikewise an adaptation of IEC TR62221, Method B (i.e., the“Reduced-Diameter Optical-Fiber Microbend Sensitivity Test”). For thismodified IEC fixed-diameter sandpaper drum test, a 300-millimeterdiameter quartz cylinder was wrapped with adhesive-backed, 320-gritsandpaper (i.e., approximately equivalent to 36-micron-grade sandpaper)to create a rough surface. Then, each 440-meter fiber sample was woundin a single layer at about 1,470 mN (i.e., a controlled tension of 150gf on the 300-millimeter diameter quartz drum using a Delachaux opticalfiber winding apparatus). Two hours after winding, fiber attenuation wasmeasured at room temperature (i.e., 23° C.) using an optical time domainreflectometer (OTDR).

Absolute fiber attenuation measured at a wavelength of 1550 nanometersis provided (below) in Table 4.

TABLE 4 (Microbend Sensitivity - 1550 nm) Optical Fiber glass fiberw/primary coating 23° C. Ex. (glass fiber and coating diameters) (dB/km)nominal 200-micron bend-insensitive SMFs with low-modulus primarycoatings A BendBright^(XS) ® w/DP1014XS 0.97 (125μ/153μ/194μ) BBendBright^(XS) ® w/DP1014XS 0.98 (125μ/154μ/197μ) C BendBright^(XS) ®w/DP1014XS 1.05 (125μ/154μ/198μ) D BendBright^(XS) ® w/DP1014XS 0.74(125μ/158μ/200μ) E BendBright^(XS) ® w/DP1014XS 0.70 (125μ/160μ/201μ)242-micron enhanced SMFs with conventional primary coatings F ESMFw/DP1007 2.004 (125μ/190μ/242μ) G ESMF w/DP1007 1.661 (125μ/190μ/242μ) HESMF w/DP1007 1.542 (125μ/190μ/242μ) I ESMF w/DP1007 1.568(125μ/190μ/242μ) J ESMF w/DP1007 1.973 (125μ/190μ/242μ)

Table 4 (above) shows that, Draka Comteq's 125-micron BendBright^(XS)®bend-insensitive single-mode glass fiber in combination with (i) alow-modulus primary coating having an outer diameter of between about150 microns and 160 microns and (ii) a secondary coating having an outerdiameter of between about 195 microns and 200 microns providessignificantly better microbending performance compared with that ofconventional 125-micron enhanced single-mode glass fiber (ESMF) incombination with a 190-micron-diameter, conventional primary coating anda 242-micron-diameter, conventional secondary coating.

Stated otherwise, a nominal 200-micron optical fiber formed from DrakaComteq's 125-micron BendBright^(XS)® bend-insensitive single-mode glassfiber and Draka Comteq's ColorLock^(XS) brand coating system providessuperior microbending performance to that of a 242-micron, enhancedsingle-mode optical fiber (ESMF) that employs conventional primary andsecondary coatings.

Moreover, a nominal 200-micron optical fiber formed from Draka Comteq's125-micron BendBright^(XS)® bend-insensitive single-mode glass fiber andDraka Comteq's ColorLock^(XS) brand coating system provides similarmicrobending performance to that of a 242-micron, enhanced single-modeoptical fiber (ESMF) that employs a comparable low-modulus primarycoating and a comparable secondary coating. By way of example, the200-micron optical fibers designated Examples A-E in Table 4 (above)provide comparable microbending performance to that of the 242-micronoptical fiber designated Example H in Table 2 (above), which, as noted,is a 242-micron optical fiber having an enhanced standard single-modefiber (ESMF) and thicker layers of comparable low-modulus primary andsecondary coatings.

As noted, whereas single-mode glass fibers that are commerciallyavailable from Draka Comteq under the trade name BendBright® arecompliant with the ITU-T G.652.D requirements, single-mode glass fibersthat are commercially available from Draka Comteq under the trade namesBendBright^(XS)® and BendBright-Elite™ are compliant with the ITU-TG.652.D requirements and the ITU-T G.657.A/B requirements. Therespective ITU-T G.652 recommendations and the respective ITU-T G.657recommendations are hereby incorporated by reference in their entirety.

In this regard, this application incorporates by reference productspecifications for the following Draka Comteq single-mode opticalfibers: (i) Enhanced Single Mode Fiber (ESMF); (ii) BendBright®single-mode optical fiber; (iii) BendBright^(XS)® single-mode opticalfiber; and (iv) BendBright-Elite™ single-mode optical fiber. Thistechnical information is provided as Appendices 1-4, respectively, inpriority U.S. Provisional Application No. 61/248,319 for aReduced-Diameter Optical Fiber (filed Oct. 2, 2009), which, as noted, isincorporated by reference in its entirety. Table 5 (below) depictsoptical-fiber attributes of an exemplary bend-insensitive optical fiberin accordance with the present invention.

TABLE 5 (Exemplary Optical-Fiber Attributes) Attribute Detail Value ModeField Diameter Wavelength (nm) 1310 Range of Nominal 8.5-9.3 Values (μm)Cladding Diameter Nominal (μm) 125 Tolerance (μm) ±0.7 CoreConcentricity Error Maximum (μm) 0.5 Cladding Non-Circularity Maximum(%) 0.7 Cable Cut-Off Wavelength Maximum (nm) 1260 Macrobending LossRadius (mm) 15 10 7.5 Number of Turns 10 1 1 Maximum @1550 nm 0.03 0.10.5 (dB) Maximum @1625 nm 0.1 0.2 1.0 (dB) Proof Stress Minimum (GPa)0.7 Chromatic Dispersion λ_(0min) (nm) 1300 Coefficient λ_(0max) (nm)1324 S_(0max) (ps/(nm² · km)) ≦0.092

It is within the scope of the present invention to achievereduced-diameter optical fibers by employing other kinds oftrench-assisted, bend-insensitive glass fibers. In this regard, U.S.Patent Application Publication No. US 2008/0056654 A1 for a for a LowBend Loss Single-Mode Optical Fiber (Bickham et al.), which is herebyincorporated by reference in its entirety, discloses a glass fiber thatincludes a cladding region with a depressed refractive index.

Furthermore, it is within the scope of the present invention to achievereduced-diameter optical fibers by employing bend-insensitive glassfibers that include regular or random holes, whether continuous ordiscrete, in an annular region (e.g., an inner cladding). In thisregard, U.S. Pat. No. 7,444,838 for a Holey Optical Fiber with RandomPattern of Holes and Method for Making the Same (Pickrell et al.) andU.S. Pat. No. 7,567,742 for a Holey Optical Fiber with Random Pattern ofHoles and Method for Making Same (Pickrell et al.), each of which ishereby incorporated by reference in its entirety, disclose a glass fiberthat includes a holey region (e.g., a cladding) with a random array ofholes. Similarly, U.S. Pat. No. 7,450,806 for Microstructured OpticalFibers and Methods (Bookbinder et al.), which is hereby incorporated byreference in its entirety, discloses a microstructured glass fiber thatincludes voids within the cladding region.

Other trench-assisted and/or void-assisted optical fibers are disclosedin the following patents and patent application publications, each ofwhich is hereby incorporated by reference in its entirety: U.S. Pat. No.4,852,968 for an Optical Fiber Comprising a Refractive Index Trench(Reed); U.S. Pat. No. 5,044,724 for a Method of Producing Optical Fiber,and Fiber Produced by the Method (Glodis et al.); U.S. Pat. No.6,901,197 for a Microstructured Optical Fiber (Hasegawa et al.); U.S.Pat. No. 7,095,940 for an Optical Fiber, Method for Manufacturing Sameand Optical Transmission Channel (Hayami et al.); U.S. Pat. No.7,228,040 for a Hole-Assisted Single Mode Optical Fiber (Nakajima etal.); U.S. Pat. No. 7,239,784 for an Optical Fiber, Method forManufacturing Same and Optical Transmission Channel (Hayami et al.);U.S. Pat. No. 7,292,762 for a Hole-Assisted Holey Fiber and Low BendingLoss Multimode Holey Fiber (Guan et al.); U.S. Pat. No. 7,433,566 for aLow Bend Loss Optical Fiber with High Modulus Coating (Bookbinder etal.); U.S. Pat. No. 7,526,166 for a High Numerical Aperture Fiber(Bookbinder et al.); U.S. Pat. No. 7,526,169 for a Low Bend LossQuasi-Single-Mode Optical Fiber and Optical Fiber Line (Bickham et al.);U.S. Pat. No. 7,555,187 for a Large Effective Area Fiber (Bickham etal.); U.S. Pat. No. 7,450,807 for a Low Bend Loss Optical Fiber withDeep Depressed Ring (Bickham et al.); U.S. Pat. No. 7,574,088 for anOptical Fiber and Optical Fiber Ribbon, and Optical InterconnectionSystem (Sugizaki et al.); U.S. Patent Application Publication No. US2008/0166094 A1 for a Bend Resistant Multimode Optical Fiber (Bickham etal.); U.S. Patent Application Publication No. US 2008/0304800 A1 for anOptical Fiber with Large Effective Area (Bickham et al.); U.S. PatentApplication Publication No. US 2009/0060437 A1 for Bend Insensitivity inSingle Mode Optical Fibers (Fini et al.); U.S. Patent ApplicationPublication No. US 2009/0126407 A1 for Methods for Making Optical FiberPreforms and Microstructured Optical Fibers (Bookbinder et al.); U.S.Patent Application Publication No. US 2009/0154888 A1 for a BendResistant Multimode Optical Fiber (Steele et al.); U.S. PatentApplication Publication No. US 2009/0169163 A1 for a Bend ResistantMultimode Optical Fiber (Steele et al.); and International PatentApplication Publication No. WO 2009/064381 A1 for Methods for MakingOptical Fiber Preforms and Microstructured Optical Fibers (Bookbinder etal.).

It is believed that the foregoing glass fibers, as well as other glassfibers disclosed in previously incorporated-by-reference patentdocuments, might be combined with the low-modulus primary coatings asherein disclosed to achieve satisfactory, reduced-diameter opticalfibers. As such, the resulting reduced-diameter optical fibers (e.g.,holey fibers with low-modulus primary coatings) are within the scope ofthe present invention.

That said, it has been preliminarily observed that, with respect toreduced-diameter optical fibers having low-modulus primary coatings,bend-insensitive glass fibers having full-solid designs (e.g.,125-micron BendBright^(XS)® bend-insensitive single-mode glass fiber)seem to provide better microbending performance than do bend-insensitiveglass fibers having hole-assisted designs.

Furthermore, it has been preliminarily observed that, with respect toreduced-diameter optical fibers, bend-insensitive glass fibers havingfull-solid designs (e.g., 125-micron BendBright^(XS)® bend-insensitivesingle-mode glass fiber) also seem to provide better mechanicalperformance than do bend-insensitive glass fibers having void-assisteddesigns (e.g., holey fibers). Those having ordinary skill in the artwill appreciate that mechanical robustness is an important considerationwhen employing a bend-insensitive glass fiber within a nominal200-micron optical fiber.

In this regard, 200-micron optical fibers that are formed from (i) DrakaComteq's 125-micron BendBright^(XS)® bend-insensitive single-mode glassfiber, which has a full-solid glass design, and (ii) Draka Comteq'sColorLock^(XS) brand coating system demonstrate comparable mechanicalreliability to that of a standard 242-micron optical fiber (e.g., aSSMF).

The 200-micron optical fibers that are formed from Draka Comteq's125-micron BendBright^(XS)® bend-insensitive single-mode glass fiber andDraka Comteq's ColorLock^(XS) brand coating system were tested fortensile strength and dynamic fatigue in accordance with the FOTP-28standard, which is hereby incorporated by reference in its entirety.Representative mechanical reliability for these 200-micron opticalfibers, which possessed differently colored secondary coatings, isprovided (below) in Table 6.

TABLE 6 (Mechanical Reliability) Tensile Strength Tensile StrengthColorLock^(XS) 50% failure 15% failure Dynamic Fatigue color (kpsi)(kpsi) (n-value) Blue 711 539 22.5 Orange 712 626 22.0 Green 705 60020.4 Brown 675 557 20.8 Slate 721 623 22.8 White 729 577 21.8 Red 708577 20.9 Black 709 627 22.8 Yellow 715 540 21.4 Violet 713 580 21.6 Rose723 557 21.9 Aqua 730 580 23.0

As will be understood by those having ordinary skill in the art,industry minimum requirements for tensile strength at fiber failure are550 kpsi at the 50^(th) percentile of the optical-fiber tensile-strengthdistribution (i.e., the median tensile strength) and 455 kpsi at the15^(th) percentile of the optical-fiber tensile-strength distribution.

The industry minimum requirement for the dynamic fatigue stresscorrosion factor (n-value) is 18. In this regard, dynamic fatigue stresscorrosion factor provides an indication of how fast a flaw in the glassfiber's silica structure propagates under strain.

As will be further understood by those having ordinary skill in the art,for both tensile strength and dynamic fatigue stress corrosion factor,an adequate sampling of optical fibers (e.g., n=30) provides astatistical estimate that facilitates characterization the optical-fiberpopulation.

In another alternative embodiment, the outer diameter of the componentglass fiber may be reduced to less than 125 microns (e.g., between about60 microns and 120 microns), perhaps between about 70 microns and 115microns (e.g., about 80-110 microns). This may be achieved, forinstance, by reducing the thickness of one or more cladding layers.

As compared with the prior alternative embodiment, (i) the totaldiameter of the optical fiber may be reduced (i.e., the thickness of theprimary and secondary coatings are maintained in accordance with theprior alternative embodiment) or (ii) the respective thicknesses of theprimary and/or secondary coatings may be increased relative to the prioralternative embodiment (e.g., such that the total diameter of theoptical fiber might be maintained).

By way of illustration, with respect to the former, a component glassfiber having a diameter of between about 90 and 100 microns might becombined with a primary coating layer having an outer diameter ofbetween about 110 microns and 150 microns (e.g., about 125 microns) anda secondary coating layer having an outer diameter of between about 130microns and 190 microns (e.g., about 155 microns). With respect to thelatter, a component glass fiber having a diameter of between about 90and 100 microns might be combined with a primary coating layer having anouter diameter of between about 120 microns and 140 microns (e.g., about130 microns) and a secondary coating layer having an outer diameter ofbetween about 160 microns and 230 microns (e.g., about 195-200 microns).

It seems that reducing the diameter of the component glass fiber mightmake the resulting optical fiber more susceptible to microbendingattenuation. For example, as compared with a component glass fiberhaving a standard diameter of 125 microns, a component glass fiberhaving a diameter of 110 microns might be twice as susceptible tomicrobending losses. That said, the advantages of further reducingoptical-fiber diameter may be worthwhile for some optical-fiberapplications.

In view of the foregoing, commonly assigned U.S. Patent Application No.61/177,996 for a Reduced-Diameter Optical Fiber, filed May 13, 2009,(Overton) and U.S. Patent Application No. 61/248,319 for aReduced-Diameter Optical Fiber, filed Oct. 2, 2009, (Overton) are herebyincorporated by reference in their entirety.

* * *

As noted, the optical fiber according to the present invention mayinclude one or more coating layers (e.g., a primary coating and asecondary coating). At least one of the coating layers—typically thesecondary coating—may be colored and/or possess other markings to helpidentify individual fibers. Alternatively, a tertiary ink layer maysurround the primary and secondary coatings.

As discussed previously, combining (i) a coating system according to thepresent invention with (ii) a glass fiber having a refractive indexprofile that itself provides bend resistance (e.g., low macrobendingsensitivity) has been found to provide unexpectedly superior reductionsin microbend sensitivity. Indeed, bend-insensitive glass fibers areespecially suitable for use with the coating system of the presentinvention (e.g., Draka Comteq's ColorLock^(XS) brand coating system).

* * *

Accordingly, the optical fiber (e.g., bend-insensitive optical fiber) asherein disclosed may be included in all-dielectric self-supporting(ADSS) cables that tolerate increased cable strain while maintaining oreven reducing fiber strain.

All-dielectric self-supporting (ADSS) cables are commonly used toupgrade existing high-voltage power lines with optical fibers. Those ofordinary skill in the art will appreciate that ADSS cables are usuallyconstructed exclusively from dielectric (i.e., nonconductive) materials.Moreover, ADSS cables are designed to withstand relatively high cablestrain—often caused by wind and/or ice accumulation—while maintaininglow strain (e.g., less than about 0.05 percent) on the optical fiberswithin the ADSS cable.

Strain window refers to the axial load that can be applied to a cable(e.g., an ADSS cable) before more than negligible amounts of strain(i.e., elongation) are imparted to the optical fibers within the cable.More specifically, the strain window can be defined as the percent axialelongation the cable can experience before the optical fibers experiencefiber strain. Increasing the strain window is desirable because itallows additional stress to be imparted to the fiber optic cable withoutstraining the optical fibers.

The strain window has two components, namely stranding strain window andexcess fiber length (EFL).

The stranding strain window is described by the difference betweenoptical fiber length and cable length due to the stranding of buffertubes containing the optical fibers. As will be appreciated by those ofordinary skill in the art, buffer tubes (and optical fibers enclosedtherein) that are stranded within a cable (e.g., helically strandedaround a central strength member) have a total length that exceeds thelength of the cable itself. In this respect, those having ordinary skillin the art will understand that the stranding strain window depends uponcertain design variables, such as buffer-tube inner diameter,buffer-tube outer diameter, central-strength-member diameter,fiber-bundle diameter, and stranding lay length. Conventional ADSScables typically have a stranding strain window of at least about 0.65percent. That said, ADSS cables having a larger (or smaller) strandingstrain window are within the scope of the present invention.

EFL is the relative difference between the actual individual fiberlength and the length of the buffer tube containing the optical fiber.EFL is typically expressed as a percentage, namely ((length of opticalfiber−length of buffer tube)/length of buffer tube)×100. Typically, allof the optical fibers within the present ADSS cables have approximatelythe same EFL.

Increasing the strain window by increasing EFL is problematic,particularly for conventional optical fibers, in that EFL can causepartial buckling of the optical fiber when the cable is in an unstrainedstate. Contraction of the cable and/or buffer tubes (e.g., due to coldtemperatures) may cause further buckling of the optical fibers. Bucklingmay result in microbending, which can increase the fiber attenuation.Therefore, increasing the strain window of conventional ADSS cables byincreasing the stranding strain window is preferable as it reduces therisk of optical-fiber buckling. In this regard, conventional ADSS cablesare designed to have excess fiber length of approximately zero.Accordingly, the strain window of a conventional ADSS cable isapproximately equal to the stranding strain window of the ADSS cable.

There is, however, a limit to how much the stranding strain window canbe increased. Additionally, increasing the stranding strain window mayrequire a cable having a larger, and potentially more expensive,structure. For example, the stranding strain window can be increased byincreasing the diameter of the cable's central strength member orincreasing the buffer tube inner or outer diameter, or otherwisedecreasing stranding lay length (i.e., the longitudinal distance alongan ADSS cable in which stranded buffer tubes complete one helical wrap).

Thus, a need exists for ADSS cables having a greater strain window witha lower risk of cold temperature failure.

Accordingly, one or more bend-insensitive optical fibers according tothe present invention may be incorporated into an ADSS cable. Suchbend-insensitive fibers enable the ADSS cables to exhibit excellentattenuation performance, even in low temperature environments.

The ADSS cable may contain a plurality of loose buffer tubes. In aparticular embodiment, the ADSS cable contains six loose buffer tubes.Each loose buffer tube encloses one or more bend-insensitive opticalfibers as disclosed hereinabove. The bend-insensitive optical fiberswithin each buffer tube may be substantially longer than the buffertubes, thereby resulting in a high EFL. A high EFL is feasible because,as compared to standard optical fibers, the present bend-insensitiveoptical fibers exhibit excellent microbending and attenuationperformance.

Therefore, the optical fibers contained within the present ADSS cablesmay have an EFL of about 0.1 percent or more, typically at least about0.2 percent (e.g., at least about 0.3 percent). Accordingly, the presentADSS cables may have a strain window greater than the stranding strainwindow of the ADSS cables. For example, an ADSS cable in accordance withthe present invention may have stranding strain window of about 0.7percent and EFL of about 0.3 percent, resulting in a strain window ofabout 1.0 percent.

With respect to the present ADSS cables, the component buffer tubes maybe formed of polyolefins (e.g., polyethylene, polypropylene, or blendsthereof), including fluorinated polyolefins, polyesters (e.g.,polybutylene terephthalate), polyamides (e.g., nylon), as well as otherpolymeric materials and polymeric blends.

The buffer tubes may be stranded (e.g., helically) around a centralstrength member. Indeed, the buffer tubes may be stranded around thecentral member in such a way so that the stranding strain window isrelatively small. For example, the stranding strain window could bereduced by decreasing the diameter of the central strength member or bydecreasing the diameter of the buffer tubes. Alternatively, thestranding strain window could be maintained at conventional levels.

The ADSS cable may have one or more sheath layers that may enclosestrength yarns (e.g., aramid yarns) and/or ripcords (i.e., used foraccessing the interior of the cable). The ADSS cable outer sheath may beformed from polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluoroethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

FIG. 13 depicts an exemplary ADSS cable 20 in accordance with thepresent invention. The ADSS cable 20 includes six buffer tubes 21, eachof which contains a plurality of optical fibers 22 (e.g., six opticalfibers). The buffer tubes 21 are stranded about a central strengthmember 23, which is centrally positioned within a cable jacket 24. TheADSS cable 20 further includes a layer of aramid yarns 25 surroundingthe buffer tubes 21. Ripcords 26 are positioned within the ADSS cable 20to facilitate cable access.

The ADSS cables according to the present invention provide advantagesbecause they may possess high EFL while either retaining or reducingstranding strain window. As compared with typical ADSS cables, if EFL isincreased relatively more than the stranding strain window is decreased,then the ADSS cables according to the present invention will have animproved strain window as compared to typical ADSS cables.

ADSS cables according to the present invention are advantageous in theirpotential for manufacturing cost savings. For example, if the ADSS cablehas an improved strain window, then the required amount of strengthmaterials (e.g., aramid yarn) can be decreased, thereby savingmanufacturing costs. Additionally, if the stranding strain window isreduced as described previously, then smaller cables structures (e.g.,employing a smaller diameter central strength member or smaller diameterbuffer tubes) may be employed, achieving cost savings. Smaller cablestructures have the added advantage of reducing strain that can becaused by wind and/or ice accumulation.

In this regard and by way of example, buffer tube diameters may bereduced by about 15-20 percent, and cable diameters may be reduced byabout 10-15 percent while maintaining the same strain window. By way offurther example, the outer diameter of an exemplary buffer tube may bereduced from about 3.0 millimeters to about 2.5 millimeters. By way ofeven further example, increasing the strain window from about 0.7percent to about 1.0 percent (e.g., by having EFL of about 0.3 percent)can result in a nearly 50 percent reduction in the amount of strengthmaterials (e.g., aramid yarns) needed in the cable to withstand typicalweather-related loading (e.g., about 1680 pounds of tension). Thosehaving ordinary skill in the art will appreciate that the requirednumber of strength elements depends on their corresponding tensilemodulus.

* * *

This application further incorporates entirely by reference thefollowing commonly assigned patents, patent application publications,and patent applications: U.S. Pat. No. 5,574,816 forPolypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,717,805 forStress Concentrations in an Optical Fiber Ribbon to FacilitateSeparation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362 forPolypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,911,023 forPolyolefin Materials Suitable for Optical Fiber Cable Components; U.S.Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No.6,066,397 for Polypropylene Filler Rods for Optical Fiber CommunicationsCables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon andMethod for Making the Same; U.S. Pat. No. 6,085,009 for Water BlockingGels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes andCables Made Therewith; U.S. Pat. No. 6,215,931 for FlexibleThermoplastic Polyolefin Elastomers for Buffering Transmission Elementsin a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method forAccessing Optical Fibers in the Midspan Region of an Optical FiberCable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbonand Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method forAccessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section;U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix MaterialHaving Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for anOptical Fiber Having Water Swellable Material for Identifying Groupingof Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for ManufacturingOptical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene FillerRods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491for an Optical Drop Cable for Aerial Installation; U.S. Pat. No.7,346,244 for a Coated Central Strength Member for Fiber Optic Cableswith Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skinfor Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube thatResults in Easy Access to and Low Attenuation of Fibers Disposed WithinBuffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-SpeedGel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446for an Optical Fiber Cable with Cushion Members Protecting Optical FiberRibbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus toReduce Variation of Excess Fiber Length in Buffer Tubes of Fiber OpticCables; U.S. Pat. No. 6,618,538 for a Method and Apparatus to ReduceVariation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables;U.S. Pat. No. 7,322,122 for a Method and Apparatus for Curing a FiberHaving at Least Two Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347for an Optimized Fiber Optic Cable Suitable for Microduct BlownInstallation; U.S. Pat. No. 6,941,049 for a Fiber Optic Cable Having NoRigid Strength Members and a Reduced Coefficient of Thermal Expansion;U.S. Pat. No. 7,162,128 for Use of Buffer Tube Coupling Coil to PreventFiber Retraction; U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton etal.); U.S. Patent Application Publication No. 2008/0292262 for aGrease-Free Buffer Optical Fiber Buffer Tube Construction Utilizing aWater-Swellable, Texturized Yarn (Overton et al.); European PatentApplication Publication No. 1,921,478 A1, for a TelecommunicationOptical Fiber Cable (Tatat et al.); U.S. Pat. No. 7,570,852 for anOptical Fiber Cable Suited for Blown Installation or PushingInstallation in Microducts of Small Diameter (Nothofer et al.); U.S.Patent Application Publication No. US 2008/0037942 A1 for an OpticalFiber Telecommunications Cable (Tatat); U.S. Pat. No. 7,599,589 for aGel-Free Buffer Tube with Adhesively Coupled Optical Element (Overton etal.); U.S. Pat. No. 7,567,739 for a Fiber Optic Cable Having aWater-Swellable Element (Overton); U.S. Patent Application PublicationNo. US2009/0041414 A1 for a Method for Accessing Optical Fibers within aTelecommunication Cable (Lavenne et al.); U.S. Patent ApplicationPublication No. US2009/0003781 A1 for an Optical Fiber Cable Having aDeformable Coupling Element (Parris et al.); U.S. Patent ApplicationPublication No. US2009/0003779 A1 for an Optical Fiber Cable HavingRaised Coupling Supports (Parris); U.S. Patent Application PublicationNo. US2009/0003785 A1 for a Coupling Composition for Optical FiberCables (Parris et al.); U.S. Patent Application Publication No.US2009/0214167 A1 for a Buffer Tube with Hollow Channels, (Lookadoo etal.); U.S. patent application Ser. No. 12/466,965 for an Optical FiberTelecommunication Cable, filed May 15, 2009, (Tatat); U.S. patentapplication Ser. No. 12/506,533 for a Buffer Tube with AdhesivelyCoupled Optical Fibers and/or Water-Swellable Element, filed Jul. 21,2009, (Overton et al.); U.S. patent application Ser. No. 12/557,055 foran Optical Fiber Cable Assembly, filed Sep. 10, 2009, (Barker et al.);U.S. patent application Ser. No. 12/557,086 for a High-Fiber-DensityOptical Fiber Cable, filed Sep. 10, 2009, (Lovie et al.); U.S. patentapplication Ser. No. 12/558,390 for a Buffer Tubes for Mid-Span Storage,filed Sep. 11, 2009, (Barker); U.S. patent application Ser. No.12/614,692 for Single-Fiber Drop Cables for MDU Deployments, filed on orabout Nov. 9, 2009, (Overton); U.S. patent application Ser. No.12/614,754 for Optical-Fiber Loose Tube Cables, filed on Nov. 9, 2009,(Overton); U.S. patent application Ser. No. 12/615,003 for aReduced-Size Flat Drop Cable, filed on Nov. 9, 2009, (Overton); U.S.patent application Ser. No. 12/615,698 for Reduced-Diameter RibbonCables with High-Performance Optical Fiber, filed on or about Nov. 10,2009, (Overton); U.S. patent application Ser. No. 12/615737 for aReduced-Diameter, Easy-Access Loose Tube Cable, filed on or about Nov.9, 2009, (Overton); U.S. Patent Application No. 61/112,845 forSingle-Fiber Drop Cables for MDU Deployments, filed Nov. 10, 2008,(Overton); U.S. Patent Application No. 61/112,863 forBend-Insensitive-Fiber Loose Tube Cables, filed Nov. 10, 2008,(Overton); U.S. Patent Application No. 61/112,912 for a Reduced-SizeFlat Drop Cable with Bend-Insensitive Fiber, filed Nov. 10, 2008,(Overton); U.S. Patent Application No. 61/112,926 for ADSS Cables withBend-Insensitive Fiber, filed Nov. 10, 2008, (Overton); U.S. PatentApplication No. 61/112,965 for Reduced-Diameter Ribbon Cables withHigh-Performance Optical Fiber, filed Nov. 10, 2008, (Overton); U.S.Patent Application No. 61/113,067 for a Reduced-Diameter, Easy-AccessLoose Tube Cable, filed Nov. 10, 2008, (Overton).

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In the specification and/or figures, typical embodiments of theinvention have been disclosed. The present invention is not limited tosuch exemplary embodiments. The figures are schematic representationsand so are not necessarily drawn to scale. Unless otherwise noted,specific terms have been used in a generic and descriptive sense and notfor purposes of limitation.

1. An ADSS cable, comprising: a polymeric cable jacket; and one or morebuffer tubes positioned within said polymeric cable jacket, wherein atleast one of said buffer tubes encloses an optical fiber; wherein saidoptical fiber comprises a glass fiber and a primary coating surroundingsaid glass fiber, said primary coating possessing (i) an in situ modulusof less than 0.50 MPa and (ii) a glass transition temperature of lessthan −55° C.; wherein, at a wavelength of 1310 nanometers, said opticalfiber has a mode field diameter with nominal values of between 8.5microns and 9.3 microns; wherein, said optical fiber has a standardcable cut-off wavelength of no more than 1260 nanometers; wherein, saidoptical fiber has a zero chromatic dispersion wavelength of at least1300 nanometers and no more than 1324 nanometers; wherein, at the zerochromatic dispersion wavelength, said optical fiber has a slope of nomore than 0.092 ps/(nm²·km); wherein, at a wavelength of 1550nanometers, said optical fiber has induced bending attenuation of (i)0.03 dB or less for ten turns around a mandrel radius of 15 millimeters,(ii) 0.1 dB or less for one turn around a mandrel radius of 10millimeters, and (iii) 0.5 dB or less for one turn around a mandrelradius of 7.5 millimeters; wherein, at a wavelength of 1625 nanometers,said optical fiber has induced bending attenuation of (i) 0.1 dB or lessfor ten turns around a mandrel radius of 15 millimeters, (ii) 0.2 dB orless for one turn around a mandrel radius of 10 millimeters, and (iii)1.0 dB or less for one turn around a mandrel radius of 7.5 millimeters;and wherein said cable is substantially free of conductive materials. 2.An ADSS cable according to claim 1, comprising a central strength membercentrally positioned within said polymeric cable jacket.
 3. An ADSScable according to claim 2, wherein at least one of said buffer tubes isstranded around said central strength member.
 4. An ADSS cable accordingto claim 1, wherein said primary coating defines a primary coating layerhaving an outer diameter between 135 microns and 175 microns.
 5. An ADSScable according to claim 1, wherein said optical fiber has an outerdiameter of between 150 microns and 230 microns.
 6. An ADSS cableaccording to claim 1, wherein said optical fiber has an outer diameterof less than about 200 microns.
 7. An ADSS cable according to claim 1,wherein said optical fiber is a single-mode optical fiber.
 8. An ADSScable according to claim 1, wherein said primary coating possesses an insitu modulus of more than about 0.2 MPa.
 9. An ADSS cable according toclaim 1, wherein said primary coating possesses an in situ modulus ofbetween about 0.3 MPa and 0.4 MPa.
 10. An ADSS cable according to claim1, wherein said primary coating possesses a glass transition temperatureof less than about −60° C.
 11. An ADSS cable according to claim 1,wherein said optical fiber has an excess-fiber length of at least about0.1 percent.
 12. An ADSS cable according to claim 1, wherein saidoptical fiber has an excess-fiber length of at least about 0.2 percent.13. An ADSS cable according to claim 1, wherein said optical fiber hasan excess-fiber length of at least about 0.3 percent.
 14. An ADSS cableaccording to claim 1, wherein the ADSS cable has a strain window thatexceeds the stranding strain window of the ADSS cable.
 15. An ADSS cableaccording to claim 1, wherein the ADSS cable has a strain window thatexceeds the stranding strain window of the ADSS cable by at least about0.2 percent.
 16. An ADSS cable according to claim 1, wherein saidprimary coating possesses an in situ modulus of between 0.2 MPa and 0.4MPa.
 17. An ADSS cable according to claim 1, wherein said primarycoating possesses (i) an in situ modulus of between 0.3 MPa and 0.4 MPaand (ii) a glass transition temperature of less than −60° C.